Cellular communication systems are known. Such systems are, typically, comprised of a number of cells, each having a service coverage area, and a number of cellular telephones (mobile stations, MS). The service coverage areas of adjacent cells may be arranged to partially overlap in such a manner as to provide a substantially continuous coverage area in which a mobile station receiving service from one cell may be handed off to an adjacent cell with no interruption in service. The Groupe Special Mobile (GSM) Pan-European digital cellular system, as specified in GSM recommendations available from the European Telecommunications Standards Institute (ETSI), is an example of just such a system.
A cell's radio coverage is provided by a base-station (BS). Each BS contains one or more transceivers (TRX) which can simultaneously receive on one frequency and transmit on another. Communication between a BS and a MS typically occurs using a portion of a pair of frequencies (transmit and receive) temporarily assigned in support of the communication transaction at the BS.
The pair of frequencies assigned for use at the remote site are typically referred to as a radio channel. Downlink transmissions from a BS to a MS on the radio channel occur on a first frequency of the pair of frequencies. Uplink transmissions from an MS to a BS on the radio channel occurs on the second frequency of the pair of frequencies.
The GSM system is a TDM/TDMA system providing eight full duplex signal paths (8 TDM slots per TDM frame) on each radio channel. A single, primary radio channel allocated to a BS, by virtue of its being time multiplexed, can support up to seven full rate duplex traffic (speech or data) users in addition to a multiplexed common control channel within the eight TDM slots. Additional, secondary radio channels assigned to the same cell can provide a full complement of eight full rate traffic users (in the 8 TDM slots) per radio channel, since the control channel within the primary radio channel can control allocation of communication resources on secondary radio channels.
Transmissions (control, speech, and/or data traffic) from a BS to a MS occupy a first TDM slot (i.e., downlink slot) on a first frequency of a radio channel and transmissions from a MS to a BS occupy a second TDM slot (i.e., uplink slot) on the second frequency of the radio channel. The MS's uplink slot on the second frequency is displaced in time three TDM slot positions following the downlink slot on the first frequency. The MS's uplink slot on the second frequency is offset 45 MHz lower in frequency than the downlink.
Due to the aberrations caused by the mobile radio channel, various means have been employed to improve the transmission of speech signals over the radio. For one, redundancy is provided to the encoded speech information by the use of error correction coding techniques. This includes block and convolutional coding techniques.
Another improvement that has been utilized in GSM is the use of interleaving of the information and slow frequency hopping. In frequency hopping, the coded information is transmitted in sequential bursts on a multiplicity of radio frequencies. Frequency hopping systems are well known. Depending on the method of frequency hopping employed within a cell and amongst other cells, various degrees of immunity to fading and interference can be obtained. For example, the frequency hopping provides some frequency diversity in the radio channel. That is, it reduces the likelihood that the signal remains faded for an unnecessarily long period of time. If only some of the information is lost by being transmitted on frequencies that are experiencing fading, the error correction decoding still allows the information to be reproduced.
Frequency hopping can also provide a degree of robustness to interference from other users in adjacent cells. With simple frequency division multiplex, a cochannel interferer might be present that could continuously interfere with a particular mobile station. Employing uncorrelated frequency hopping sequences for the two mobile stations reduces that potential interference to only when collisions occur. This can be made relatively infrequent. Error correction coding allows information recovery even with the existence of the collisions.
With regard to the usage of frequencies within a given cell by the set of frequency hopping mobile stations, there are generally two frequency hopping assignment strategies which dictate how the selection of frequencies occurs. A first strategy is "random hopping." With "random hopping" each user has his own personal frequency sequence. The frequency sequence used to serve a particular mobile station within a cell is uncorrelated with the sequence of frequencies used to serve any other particular mobile station. Thus, there is a finite probability that a collision (two mobile stations simultaneously using the same frequency) will occur. Generally, this greatly limits the number of sequences that can be simultaneously used in a given cell.
A second frequency hopping assignment strategy is termed "orthogonal hopping", which is implemented in GSM. With "orthogonal hopping", the frequency sequences for the mobile stations served by a given cell are deterministically selected such that no user is utilizing the same frequency at the same instant of time. In this manner, there is no possibility for a collision between two mobile stations to occur. A significant performance improvement is realized as compared with non-hopped operation both from multipath (fading) mitigation and inter-cell co-channel interference.
A significant limitation with orthogonal hopping is that there is a limit to the instantaneous number of mobile stations that can be accommodated by a cell. Namely, the maximum number of mobile stations that can be served is equal to the available number of frequencies assigned for use in the cell that the mobile stations can hop over. This limitation is generally established by the limit in overall spectrum available, the reuse distance, and the reuse pattern. Further, because of the dynamics of a cellular radio system, it is generally undesirable to completely assign all the available hopping sequences within a cell. This is because there must be some sequences left as a buffer to accommodate mobile stations that make new call requests or hand over to the cell from an adjacent cell. The grade of service, or probability of blocking a mobile station from a cell, dictates how heavily the frequencies at a cell can be used. In essence, capacity must be held in reserve to meet certain peaks in demand which arise from time to time. For a fixed frequency allocation and desired grade of service, this has the effect of reducing the ultimate capacity.
Accordingly, there exists need to allow an increase in the mobile station capacity in a frequency hopping system that can be served with a fixed number of frequencies without greatly decreasing the performance, grade of service, nor increasing the complexity of the system.